Obesity in the United States – Dysbiosis from Exposure to Low-Dose Antibiotics? suggests that the obesity epidemic is driven by population-wide exposure to residual antibiotics from livestock cultivation, and the resulting impact on gut microbiota.
This is one of the most similar proposals to the theory presented in A Chemical Hunger, though they don’t go quite as far as we do, still attributing some of the influence to diet and exercise: “Most reports attribute the obesity epidemic to factors such as excess food energy intake, changes in diet and eating behavior, and increasing sedentary life style. Undoubtedly, these factors contribute, but can they all account for the rapid increase in this problem that occurred over the last two decades?”
They make a pretty compelling case. A large percentage of antibiotics are excreted in animal waste and end up in the water supply, where they affect natural microorganisms. Relevant to our interests, antibiotics are more and more prevalent in rivers as they make their way towards the ocean: “The only site at which no antibiotics were detected,” they write, “was the pristine site in the mountains before the river had encountered urban or agricultural landscapes. By the time the river had exited the urban area, 6 of the 11 antibiotic compounds that were monitored were found in the samples. At Site 5, which had both urban and agricultural influences all five of the TCs monitored were detected.”
Exposure has increased in the US over time, closely matched with the increasing prevalence of obesity — “practically overlapping with the counties with the highest obesity prevalence in the US.” Similar trends can be observed in other countries. For example, in Great Britain, by 1958, around 50% of British pigs were fed antibiotics, and nearly all piglets were given food containing tetracyclines. In West Germany in the 1960s, an estimated 80% of feeds for pigs, calves, and poultry contained antibiotics.
The Effects on Human Health of Subtherapeutic Use of Antimicrobials in Animal Feeds opens by saying, “The food-producing animal and poultry industries have undergone a dramatic change that began around 1950.” That’s a little earlier than we would expect, but depending on how you measure things, it took until the 70’s or 80’s before things really got rolling.
In meat animals, antibiotics often lead to weight gain, sometimes as high as 40% weight gain compared to control, and there’s reason to suspect that this might be linked to the microbiome. Gut microbiota influence energy intake and body weight in mammals, and even short courses of antibiotics can reduce gut microbiota and increase BMI in humans (though the BMI effect was only seen in some antibiotics).
There’s even a study where they put fecal matter from human twins into germfree mice. (This is one of the more creative study designs we’ve seen.) They started by finding pairs of twins where one twin was fat and the other twin was lean. This is pretty uncommon — normally, twins weigh the same amount. They transplanted fecal matter from the twins into mice and found that mice that got fecal matter from the obese twin gained weight — unless it was housed with one of the mice who got fecal matter from the lean twin.
However, there is also evidence against this picture. For one thing, Germany, Spain, Italy, and Japan all use a lot of antibiotics in their meat, and none of these countries is particularly obese. Australia and South Africa are both pretty obese, but both of these countries use less antibiotics than usual. This could maybe be reconciled if these countries use different kinds of antibiotics, but we would need to see that case made to evaluate it.
There’s also some evidence in favor of this theory that this paper didn’t review.
For one thing, people who eat fewer animal products have lower BMIs, and the effect seems to be dose-dependent. In a sample from 2002-2006, average BMI was lowest in vegans (23.6) and incrementally higher in ovo-lacto vegetarians (25.7), pescitarians (26.3), semi-vegetarians (27.3), and nonvegetarians (28.8). We can note that the BMI for vegans is about the same as that found in hunter-gatherers and in Civil War veterans in the 1890s. That said, everyone in this sample was a Seventh-Day Adventist, so they may not be all that representative.
India and Japan are the least obese of the developed countries. Both have obesity rates below 5%. India is the most vegetarian country on the planet and Japan, while not especially vegetarian, mostly consumes seafood in place of meat products.
This would mean that vegan diets would work really well for weight loss, right? Well, maybe. As we previously reviewed, all diets seem to work a little, and no diet seems to work all that well. We see something similar in vegetarian and vegan diets. A 2015 meta-analysis found that people assigned to vegetarian diets lost more weight than those assigned to nonvegetarian diets. People on vegan diets lost a little more weight than people on vegetarian diets, about 5.5 pounds (2.5 kg) to 3.3 pounds (1.5 kg). The studies differed quite a bit in the size of the effect, but all of them had similar conclusions. The other meta-analysis from 2015 found the same general pattern, and individual studies comparing different types of vegetarian and vegan diets seem to confirm this dose-dependent trend.
This looks a lot like other studies, where the differences between diets are technically reliable but so small as to be basically meaningless, but the possible dose-dependent effect is interesting.
The most interesting study might be this one, that compared a vegan diet to a conventional low-fat diet. So far so standard, but unlike most diet studies, which end after 12 or 18 months, this one followed up two years later. The vegan group not only lost more weight (4.9 kg versus 1.8 kg), they kept it off better at the two-year followup (3.1 kg versus 0.8 kg). On most diets people lose a little weight but then gain it right back, so the fact that people kept most of the weight off for two years is interesting. Even so, the amount of weight lost in an absolute sense is still quite small. It could take more than two years on a vegan diet for you to see all the effects — but if this were the case, you’d think people would have lost even more weight by year two, but that’s not what we see.
None of these are smoking guns. At best, they are consistent with the idea that some of these contaminants are more prevalent in animal-based foods. And we know that this can’t be about the animal products themselves, because hunter-gatherers and our ancestors in 1890 ate lots of meat and didn’t experience modern levels of obesity.
Environmental contaminants tend to build up in animals through the plants they eat, so any contaminants in the environment will bioaccumulate, and concentrations will be higher in animals than in groundwater or in plants. Compounds in a farmer’s fields will end up in the corn or alfalfa fed to their cows, and the cows will end up getting an even larger dose, which will be passed on to the person who eats the resulting cheeseburger. So the fact that meat consumption is linked to obesity doesn’t necessarily implicate antibiotics. It could be something else in the meat.